As the complexity and density of semiconductor devices increases, the need for new packaging techniques has also arisen. A multi-chip module (MCM) containing multiple devices in a single package is one packaging approach. In an MCM, multiple semiconductor and passive devices are placed on substrate(s) in an MCM package. Sealing the MCM protects the devices within the MCM. This technique achieves a higher degree of device density than separately packaged semiconductor devices mounted on a printed circuit board. The higher density of MCM's translates to faster data processing times within the MCM. Additionally, locating all of the devices within a single MCM eliminates the need for driver circuits that would otherwise be required between the various devices. Using an MCM in turn reduces the chip count and power consumption required versus using individually packaged devices. The reduced chip count of an MCM also results in an increase in reliability of a system using MCMs over using discrete devices mounted to a printed circuit board.
While MCMs solve a number of design problems, they also give rise to a new set of problems. In particular, there are concerns with the testability and access to the individual devices in an MCM. Typically, only a portion of the inputs and outputs of the individual devices in an MCM have a direct connection to the leads of the MCM. Therefore, once an MCM is populated with various semiconductor and passive devices, testing the individual devices within the MCM is difficult. The small size of the individual devices in the MCM, coupled with the fact that not all of the inputs and outputs of the individual devices in an MCM can be connected to the input/output (I/O) pins of the MCM, denies access to the individual devices within the MCM. Thereby, making troubleshooting a malfunctioning MCM very difficult. In particular, determining the inoperable device(s) in an MCM can be very difficult.
Past approaches to solve this problem include a rigid incoming inspection system to ensure placing only good devices in the MCM. Unfortunately, a full functional test of a semiconductor die is often not possible. To fully exercise semiconductor die requires testing the die over the full temperature range of its application. Additionally, achieving a good electrical contact to the bond pads of a die by test probe is difficult. This prevents the necessary testing at incoming inspection, and particularly denies testing at the full speed and over the full temperature range of the individual devices in their application in the MCM.
Once an individual device slips through incoming inspection and is placed in the MCM, the time to isolate a die, if possible at all, and the time necessary to remove and replace a bad die is very expensive and time consuming. Troubleshooting an unsealed malfunctioning MCM includes testing with electrical probes. In this approach, a test probe is used to electrically access a suspected bad device in the MCM. Disadvantages of this method include the small size of the individual devices in the MCM, the relative size of the probe in relation to the bond pads of the individual devices, the density of the devices in the MCM, the likelihood of causing damage with a misplaced probe, and the inability to fully exercise an individual device with electrical test probes. For example, to fully test a microprocessor in an MCM requires sufficient number of test probes for both data and address bus lines in the microprocessor.
For modern microprocessors the number of probes that would be required will not physically fit into the MCM cavity. Additionally, the inability to fully test individual devices mounted within the MCM results in a reiterative process of removing suspected devices and retesting the MCM until all faults are removed. A trial and error remove and replace methodology for the individual devices in a malfunctioning MCM leads to unnecessary removal of good parts and possible damage, if not destruction, to the MCM in total. This process is very time consuming and expensive without providing acceptable results. Sealing the MCM with its lid denies even probe access to the individual devices in the MCM.
It is an object of the present invention, therefore to provide an improved method and apparatus for providing electrical access to the individual devices in an MCM without increasing the lead count or package size of the MCM package. A technical advantage of the present invention is that the MCM package includes test points and/or test pins which can be easily accessed externally to the package of the MCM by either a test probe or a test socket. The test pins and test points work in combination with the leads of the MCM package to provide greater electrical access to the individual devices within the MCM.
It is a further object of the present invention to provide an apparatus and method using the improved MCM package to allow for better troubleshooting of MCMs both before and after they are sealed, and allowing for software emulation on and programming or reprogramming of devices in the MCM.